Radio receiving system



June 23, 1936. w. R. KOCH RADIO RECEIVING SYSTEM 2 Sheets-Sheet 1 Filed May 51, 1934 I who FREQUENCY //v cm; 55 PER .s'fcorm I/Vl/E/VTOB. 1/ field/111E076,

WfmEA/EK 2 Sheets-Sheet -2 June 23, 1936. w. R. KOCH RADIO RECEIVING SYSTEM Filed May 31, 1934 IIIII & k mm kw Patented June 23,1936

onrrso STATES PATENT orsies RADIO RECEIVING SYSTEM Winfield H. Koch, Camden, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application May 31, 1934, Serial No. 728,252

9 Claims. (Cl. 250-20) The present invention relates to radio receiving It is well known that when a receiver employing systems and more particularly to receiving sysautomatic selectivity control means is tuned to a. terns of that character incorporating automatic weak signal, the selectivity curve becomes sharp selectivity control means and has for one f its and few high audio frequency components of the objects the provision of automatic tone comreceived signal are properly reproduced. The pensation in conjunction therewith whereby, with large low audio frequency component remaining variations in selectivity, the audio frequency tends to cause the final reproduction of the recharacteristic of the signal output from the received signal to sound heavy with a preponderceiving system may be properly balanced for a ance of bass notes for the reason that the acoustic Cir pleasing sound effect-upon the ear. center of balance has been transferred toa low 10 With radio receiving systems embodying autoaudio frequency. matic selectivity control means, a high degree of By incorporating an automatic tone control selectivity may tend to reduce the high frequency means in a system of that character to simulend of the audio frequency response charactertaneously attenuate the low audio frequency istic to cause an over balance of low frequency components to a predetermined degree, the 15 response in the output signal characteristic which acoustic center of balance and the receiving sy swill appear to the listeners as an excess of low tem may be restored to a predetermined desired frequency or bass response when receiving weak normal v e d e reproduction, under y signals, receiving conditions, tends to be more pleasing To correct this operating characteristic, it is, in character.

therefore, an object of this invention to provide The invention will, however, be better undera tone compensating means in connection with stood from the following description when conthe automatic selectivity control means whereby d d in Connection W e accompanying said means are simultaneously controlled by and drawings a its Scope Will be pointed out in the 5 responsive to variations in signal strength. app d la ms- 1 t It is also a further object of this invention to I the rawin s, Fig re 1 is a schematic circuit provide a control system of the above character g m of an automatic Control System for adio which is responsive to received signals having an receiving apparatus and the like whereby autoamplitude above a, predetermined value, matic tone compensation is effected simul- 5w It is a still further object of the invention to taneouslywith automatic e tivity control and provide a combined automatic selectivity control which provides for dj t t of e e ee of and automatic tone control system for radio re- Control of seleotivityand tone; I ceiving apparatus and the like. Fig. 2 is a curve diagram showing the audio fre;

In automatic selectivity control systems for q y sp Characteristic of the System Of 3;, radio receiving apparatus, the control means is gu e usually applied to the high frequency signal re- 3 is a schematic circuit diagram of a receiving circuits in conjunction with automatic ceiving system embodying the control system of volume control means, and it is a further object Fig. 1; and V V of this invention to provide an improved system Fig. 4 is a portion of the circuit of Fig. 3 show- 40 of that character which includes automatic tone ing a modification of the control means embodied 40 control therewith in addition to automatic volume in Fig. 3. v control means. Referring to Fig. 1, a high frequency signal It is also an object of the present invention to conveying circuit for a radio receiving system is provide an automatic tone control system in conschematically indicated by the leads 5 and 6, the nection with a low or audio frequency amplifier latter being a return lead preferably grounded as circuit of a receiving system in combination with indicated at l. The selectivity of the receiving an automatic selectivity control system which is system is varied by varying the loadlng of a operated to simultaneously attenuate low audio coupling means in the circuit such as acoupling frequency signals in a low frequency range theretransformer 8, the tuned secondary 9 of which so of as the high frequency range thereof is atis connected substantially In parallel with the m tenuated by operation of the automatic selectivity high frequency signal circuit and 1s included n control means, thereby to maintain the acoustic the anode or plate circuit Ill of an automatic center of balance for the low or audio frequency selectivity control tube H. The return lead 6 range of signals substantially constant for all is connected with a tap l2 on a voltage divider conditions of selectivity adjustment. resistor l3, l4, I5, WhlCh lS supplied the umdirectional current through terminals [6 and I! having polarities as indicated.

The cathode l8 of the automatic selectivity control tube I l is connected with a tap l9 on the voltage divider resistor and the anode circuit In is returned to the positive terminal 16 through the lead 20.

The automatic selectivity control tube is provided with a control grid 2i which is connected through a coupling resistor 22 with the cathode l8, whereby variations in potential across the resistor 22 serves to control the anode current and the loading of the high frequency signal circuit through the coupling transformer secondary 9 thereby to vary the selectivity of the high frequency circuit. With this arrangement as the control grid 2! is made more negative, the anode current is reduced through the anode circuit In and the load is correspondingly reduced through the transformer 8 whereby the selectivity of the high frequency signal circuit is correspondingly increased.

The audio frequency signal transmission circuit of the receiving system is represented by the lead 23 and the common return lead 5. Means for attenuating the low frequency signals transmitted through the circuit may be provided by any suitable automatically controllable elements such as an audio frequency choke coil 24 in series with the anode-cathode capacity of an automatic tone control tube indicated at 25. In the present example, the cathode 26 of the tube is connected with one side of the audio frequency circuit 6 through the lead 21 while the anode 28 is connected through the choke coil 24 with the high potential side 23 of the audio frequency signal transmission circuit. It will, therefore, be seen that variations of the internal anode impedance of the tube will provide variations in the loading effect of the choke coil or impedance 24 across the audio frequency circuit.

. In the present example, the impedance of the choke coil device 24 is low to low audio frequency signals and as the anode impedance of the tube 25 is reduced, its effect across the audio frequency circuit is to attenuate the low frequency signals.

The eifect of the automatic tone control tube across the circuit is controlled by 'a control grid 29 which is connected with an automatic control circuit 30 for receiving signal controlled biasing potential. It will be seen that as the grid 29 is caused to be more negative, the internal impedance of the tube 25 will increase, thereby reducing the loading effect on the circuit 23 and an increase in the low frequencv response therethrough.

Having pointed out, by wa of example, an automatic selectivity control means for a high frequency signal circuit and an automatic tone control means for a low or audio frequency signal circuit of the receiving system, automatic volume control means is provided through the medium of one or more high frequency amplifier tubes, one of which is indicated at 3i and which may be connected with any signal circuit such as a high frequency circuit. As used in the system, the signal amplifier 3| serves to receive signals on its control grid 32 through an input circuit 33 in conjunction with the return lead 6 and to transmit the amplified signals through its output anode circuit 34, the cathode being indicated at 35 and being connected through a suitable self-bias re-. sistor 36 to the lead 6. The bias supply circuit for the amplifier 3| is indicated at 37 and is connected with the control grid 32 and the high potential signal input lead 33 through a grid resistor 38.

As is well understood, variations in the bias potential applied to an amplifier such as is indicated at 3| through a control lead 31 causes a variation in the gain therethrough and this may be utilized in connection with any of the circuits of a receiver for controlling the gain therethrough. The tube 3! is shown merely for this purpose.

The gain control, tone, and selectivity control means are placed under control of the incoming signals to vary in accordance with variations in amplitude of incoming signals through an impedan'ce device or resistor 39, connected at one end 48 with the control circuit 30 for the automatic tone control device and connected at its opposite end 4| with the control circuit 37 for the gain control amplifier. The impedance device or I resistor 39 is provided with a variable tap movable thereover such as a tap 42 through which signal controlled currents may be variably applied to said device and variably coupled with either of the control circuit 30 and 31.

In the present example, and as a preferred embodiment, the signal controlled currents are derived from an impedance or resistor 43 providing a variable source of controlling potential and the resistor section l5 of the voltage divider resistor providing a fixed controlling potential in opposition to said fn st potential in circuit with the contact 42 and a fixed mid-tap 44 in the resistor 39 and a rectifier for preventing current flow through the circuit indicated in but one direction. The rectifier, in the present example, is provided by a diode plate or anode electrode 45 in association with the cathode 45 of an amplifier device or tube 41 having a control grid 48 and an output anode 49.

The resistors 43 and I5 are included in the cathode lead of the tube and the diode plate 45 is connected with the tap 44 in the resistor 39. The variable tap 42 is connected through a potential drop producing resistor 59 with the return lead 6 thereby completing the rectifier circuit.

The anode current from the anode 49 flows through the cathode resistor 43 which is of such value that the potential drop therein normally exceeds the potential drop in the fixed source I 5. For example, in practice the potential drop through the resistor 43 may equal 110 volts normally, while the drop through the resistor is may equal volts. Through the rectifier circuit, therefore, a positive potential may thus exist on the cathode 46 with respect to the anode 45 of approximately 10 volts so as no current may flow through the rectifier circuit under such conditions.

It will, therefore, be seen that by varying the biasing potential on the grid 48 in accordance with variations in signal strength, the anode current of the device 41 may be varied through the cathode resistor 43 and, in the present example, if the bias potential on the grid 48 is caused to increase negatively with increases in signal strength, the anode current will be correspondingly reduced with a result that the potential drop in the resistor 43 will correspondingly decrease and a point will be reached from which the potential across the resistor l5 will exceed that across the resistor 43. From that point on, the positive potential on the anode 45 exceeds the potential on the cathode 45 with the result that current flows in the rectifier circuit increasingly in proportion to increases in the signal strength beyond the limiting value above pointed out.

There is thus a delayed response to "increased signal strength in the rectifier circuit and this delay may be adjusted to any predetermined value by properly relating the normal potential drop in the resistor 43 to the fixed potential provided by the resistor IS.

The flow of current through the rectifier may be taken through the resistor 39 on either side of the tap 44 by adjusting the contact 42 along the resistor 39 between the terminals 40 and 4|. With this arrangement control currents set up by the potential difference of the rectifier anode and the cathode may be caused to flow directly to the anode with the contact in the position shown or through either half of the resistor 39 therefor, to provide control potentials of differing values for supply to the two control circuits 30 and '31. For example, with the contact 42 in the position shown, as soon as a signal of sufficient amplitude is received on the grid 48 to cause an unbalance in the potential existing across the resistor 43 and the resistor l5, the resulting current flow through the resistor 50 causes an increased negative bias potential to be applied to the gain control amplifier 3| in addition to the self bias potential obtained from the resistor 36 thereby increasingly reducing the gain of the amplifier and of the receiving system in response to increased signal strength.

At the same time, the potential existing across the resistor 50 is applied through the resistor 39 and terminal 40 to the control circuit 3B and serves to increase the internal impedance of the automatic tone control tube 25, thereby improving the audio frequency signal response in the low frequency range.

In response to strong signals, it is desirable to increase the fidelity of the receiving system, and, accordingly, it is desirable to broaden the tuning of the high frequency signal circuit through operation of the automatic selectivity control tube I I. Since the potential in the control circuit 30 increases negatively in response to increased signal strength, whereas the broadening of the tuning of the high frequency signal circuit requires a decrease in negative bias on the control grid 21 of the tube II, a phase reversing means such as an amplifier tube indicated at 51 is interposed between the control circuit 30 and theautomatic selectivity control tube II. In the present example, the tube 5| may be termed an automatic selectivity control amplifier and comprises a control grid'52, an output anode 53, and a cathode 54. The cathode is connected through a cathode lead 55 in which is located a self bias resistor 56 through the return lead 6, and the control grid 52 is connected in parallel with the control grid 29 to the control circuit 30. The anode 53 is connected through the coupling resistor 22 with the tap point IS on the potential supply source, while the cathode is connected to the tap point i 2. The control grid 52, therefore, serves to control the flow of anode currents through the coupling resistor 22 and thereby controls the bias potential applied to the grid 2! of the automatic selectivity control tube I.

It will be seen that the automatic selectivity control amplifier device 5| serves to reverse the phase of the control potential derived from the control circuit 30 as applied to the control grid 2| and, at the same time, serves as a voltage amplifier whereby the control potential supplied by the circuit 30 is amplified before it is applied to the control grid of the automatic selectivity control tube H.

The operation is such therefore, that as the negative potential increases in the circuit 30 as a result-of increased signal strength, the anode current through the device 5! and through the coupling resistor 22 decreases, thereby decreasing the negative biasing potential on the grid 21 and causing an increase in the anode cur-rent through the circuit I0. As hereinbefore described, this causes the high frequency signal circuit to become more heavily loaded and broadens the tuning so that the high frequency response characteristic of the receiving system is improved.

With this control system, therefore, automatic volume control means is provided in addition. to automatic selectivity and automatic tone control means responsive to variations in signal strength above a predetermined value. It will, therefore,

be seen that in response to weak signals when the selectivity increases, the low frequency response of the system is correspondingly reduced, thereby to maintain the over-all low or audio frequency response substantially symmetrical about a midfrequency range which may be taken as the acoustic center of balance for receiving signals of any strength in conjunction with an automatic selectivity control system.

The effect of the operation of the above described control system may be seen more clearly with reference to Fig. 2 in which the curve 60 represents the high frequency end of the automatic frequency output characteristic curve for the system when the selectivity is low, in response to signals of increased strength, while the portion of the curve indicated at 6! indicates the recession of the high frequency response due to the operation of the automatic selectivity control means when receiving weak signals. T

With reference to a selected'acoustic center of balance taken at a frequency indicated by the dash and dot line 62, it will'be seen that the normal low frequency response of the system indicated by the low frequency end of the response curve 63, will provide a preponderance of low frequency signals in the output of the receiving system unlessa corresponding correction is made for the low frequency signals. In the system above described, therefore, the choke coil 24, or other suitable means, and thecontrol tube. 25 or other suitable control'means therefor, are provided with such operating characteristics that simultaneously with the increase in selectivity by the automatic selectivity control means responsive to weak signals, the low frequency response characteristic is caused to recede in the opposite direction to provide a low frequency response characteristic as indicated by the dotted portion of the curve 64 whereby the overall re sponse represented by the area bet-ween .the curves BI and 64, is substantially balanced on either side of the chosen acoustic center of balance indicated by the line 62. I

Referring'again to Fig. 1,* the impedance or resistor 39 in conjunction with the movable tap 42, serves as a control device and selectivity adjusting the response of the automatic selectivity and automatic tone control means with respect to the automatic volume control means. The resistance of the element 50 is preferably lower than the resistance in either of the arms of the device 39 and in a preferred embodiment of the invention, the resistance on either side of the tap point 44, that is, between, the tap point 44 and Cal either of the terminals 40 and 4|, is substantially 500,000 ohms, while the resistance of the series element is 100,000 ohms.

Assuming that the contact 42 is moved downwardly to the terminal 4| and, assuming that, as a result of a received signal of required strength, a control current is flowing in the rectifier circuit, the above described operation causes the current to be reduced because of the insertion in the circuit of the resistance between the terminals 4| and 44 of 500,000 ohms.

The biasing potential applied to the gain control amplifier 3| is, therefore, reduced with a result that the gain is slightly increased through the receiving system, while the biasing potential applied to the automatic selectivity control and tone control systems is increased negatively because of the insertion of the resistance between the terminals 4| and 44, which is applied to the grids 52 and 29, thereby causing the fidelity to be further increased for the same signal strengths. In other words, the signal receiving circuit is broadened in its response by additional loading through operation of the automatic selectivity control tube thereby increasing the high frequency response of the system, while the low frequency response is further increased by a further increase in the impedance of the automatic tone control tube 25.

With the contact 42 at the extreme end 4| of the resistor 39, the response characteristic of the system may be considered as broad and provides high fidelity with moderate or strong signal potential input. The increase in gain through the automatic volume controlled tube 3| is compensated by the decrease in gain caused by the additional loading of the high frequency signal circuit by the automatic selectivity control tube whereby the overall gain is now substantially changed by operation of the contact 42 in the range between the tap 44 and the terminal 4|.

Therefore, it will be seen that the effect of variations in signal strength on the automatic tone and selectivity control means may be greatly increased over the eifect upon the automatic volume control means by moving the contact 42 in the direction of the terminal 4|.

The automatic volume control means may be caused to operate with full eifect and substantially to the exclusion of the automatic selectivity and automatic tone control means by moving the contact 42 in the direction of and to the terminal 40. In this position, the potential drop in the device 50 is reduced in the same manner as when the contact is moved toward and to the terminal 4|, but, in this case, the additional bias potential set up in the section of the resistor 39 between the terminal 40 and the tap 44 is in opposition and causes the negative bias potential to be reduced 'to a relatively low value on the grids 52 and 29, thereby causing the impedance of the automatic tone control tube 25 to be decreased and the impedance of the automatic selectivity control tube to be increased for a given flow of controlling current in the rectifier circuit in response to a signal of a certain strength. In this manner the overall response of the receiving system may be sharpened or broadened, depending upon the adjustment of the contact 42. Assuming a flow of control current in the rectifier circuit as a result of a received signal, the response characteristic may be shifted from a characteristic represented by the curves -63 with the contact adjacent or at the terminal 4|, while with the contact 42 at the opposite end of the resistor 39, the response characteristic may assume the shape indicated by the joined curves (SI-64.

The device 39-42 may, therefore, be considered as a potentiometer means having its terminals connected with an automatic volume control circuit for the signal amplifier and with the automatic tone and selectivity control circuit at the opposite end and, further, having a source of signal variable current connected with the movable contact and with a second tap thereon.

It will also be noted that with the contact 42 of the potentiometer device at or adjacent the terminal 40, the signal control current flow through the section 40-44 causes a potential drop which is added to the potential drop in the fixed resistor 50 and, for a given signal strength therefor, provides an increased bias on the gain control amplifier 3| so that in that position or in the direction of the terminal 40, the contact 42 serves to rapidly control the gain with respect to the tone and selectivity. This eifective control depends, to a certain degree, upon the relative resistance of the series fixed resistor 50 and the resistance in the two arms of the potentiometer device 39. From the foregoing description, it will be seen that, as pointed out, the resistance in each arm of the potentiometer device is preferably greater than in the series resistor 50. With the resistance values given, for a given signal strength the negative control potential applied to the automatic tone and selectivity control circuit 30 will be substantially 6 times as great as the negative control potential applied to the automatic gain control circuit 3'! when the contact 42 is moved to the broad end 4| of the potentiometer 39, while with the contact 42 at the opposite or sharp end 40, the negative control potentials applied to the gain control circuit 31 will be substantially 6 times greater than the negative control potential applied to the automatic tone and selectivity control circuit 30.

It will be seen that with a single variable instrumentality, the entire receiving system may be controlled with respect to gain or volume and selectivity and low frequency response or fidelity and that for any given signal strength, the response may be adjusted to any degree between limits which may be characterized as sharp and broad. The effect upon the fidelity of the system of the control operation of the device 1-39-42 is clearly represented in the fidelity curves shown in Fig. 2. The shaft and spacing of these curves may be adjusted about any selected acoustic center of balance.

Referring now to Fig. 3, a practical embodiment of the circuit of Fig. 1 is shown in a superheterodyne receiving system in which the same reference numerals are applied to corresponding parts throughout as used in Fig. l.

The control grid 48 of the main control device 47 is supplied with a biasing potential which increases negatively with increased signal strength from a resistor connected in circuit with an additional rectifier anode electrode associated with the cathode 4B. The rectifier circuit is connected with the cathode and includes the tuned secondary 12 of an intermediate frequency coupling transformer 13. The audio frequency component of the signals received through the transformer 13 and. applied to the rectifier 1| are also applied to the control grid 48 through the connection lead indicated at 14 and amplification is conveyed through the audio frequency circuit 23 to an audio frequency amplifier 15 through a suitable input transformer 1-6 and a coupling condenser ll. Anode potential is supplied to the anode 49 through a coupling choke coil 18.

A plurality of automatic selectivity control tubes I I are provided throughout the system under control of the automatic selectivity control amplification tube 5| having coupling thereto through the resistor 22. Bias or automatic volume control potentials for the radio frequency, detector, and intermediate frequency amplifier devices 19, and 8|, corresponding to the tube 31 of Fig. 1, are supplied through the control lead 31 while controlling potentials for the automatic selectivity controlling amplifier 5i and the automatic tone control tube 82, corresponding to the tube 25 in Fig. 1, are supplied through the control lead 39. The tone control choke coil is indicated at 24 across the audio frequency circuit 236 in series with the anode-cathodeimpedance. 7

A pentode type of tube is shown at 82 as a preferred automatic tone control tube and may be of the type known commercially as RCA 58 or 57'. It is preferably a tube of that type having a low internal impedance as the control grid. is

caused to become more positive for a tube which.

has a rapid plate impedance changing in response to relatively small change in bias potential. In this manner the rapid and effective change in tone compensation is possible with relatively small changes in signal strength.

In the present example, the field 83 of the loud speaker, indicated at 84, is utilized asthe voltage drop producing element to provide a fixed potential in opposition to the potential developed in. the automatic volume control resistor 43, thereby replacing the section l5 of the voltage divider resistor of Fig. I. Inthe present example, it will further be noted that the fixed tap. point 44 of Fig. 13 for the anode 45 is replaced by a variable contact 85 whereby the ratio between the two arms of the potentiometer devices may be varied, if desired, to establish a greater-range of control for the automatic selectivity and automatic tone control circuit as compared. to the arrangement of control for the automatic volume control circuit. 7

It will further be noted that the device 41 functions as. a combined second detector, first stage audio frequency amplifier, and for automati'c volume control means, while comprising merely a simple triode amplifier and double diode in one envelope. Such devices may be provided by a commercial tube known as the RCA-55 or RCA-85.

With variations in signal strength received through the intermediate frequency amplifier coupling transformer 13', the variable bias applied to the grid 48' causes the system shown to operate in the same manner as that described in Fig. 1' and for the same purpose, the present receiving system being shown as a preferred embodiment of the invention although other control means for supplying the control currents to the control device 39 may be provided and likewise the selectivity and automatic volume: control circuits may be modified in conjunction with different receiving systems In accordance with the invention",v therefore, in connection with a receiving: system which provides for a variation in selectivity the signal receiving circuits and a resulting variation in the fidelty of the audio frequency or output signal response characteristic, there is further provided for simultaneous operation therewith, a-

tone control system in connection with the audio frequency circuit, for maintaining the audio frequency characteristic substantially uniformor balanced about a predetermined acoustic center of balance, whereby the effect upon the ear for various degrees of selectivity is not changed as the selectivity is varied.

It will be seen that in addition, means are provided for varying the relative response of the automatic selectivity and tone control means with respect to the automatic volume control means, whereby for any degree of signal strength, the response thereto may be rendered relatively broad or relatively sharp and with any degree between such limits.

In general, it may be said thatthere is interposed between two automatic control circuits, one leading to the automatic volume control means and the other leading to the automatic selectivity and tone control means, a: potentiometer device torwhich currents are supplied. for the purpose of producing potential drop therein. in accordance with variations in signal strength and the relative distribution. of such potentials is made adjustable in connection with or by the potentiometer means.

In the preferred embodiment of. the invention, as shown, the potentiometer device is caused to be included in circuit with a rectifier to which circuit a signal controlled potential is applied and, in addition, as shown, includes a source of delayed potential or an opposing potential source whereby the control action is delayed until signals of a predetermined strength are received. The delay potential may be provided by variable means and likewise the potentiometer device 39 may be provided by other means such, for example, as is shown in Fig. 4, wherein the control circuits30 and 31 are connected at opposite ends of a double potentiometer device comprising parallel connected resistor sections and 91, each having a variable contact 92 and 93', respectively. One of the contacts 93 is connected with the diode plate 45 of the device 41 while the other contact 92 is connected with the positive end of the fixed potential source indicated by the resistor 94. The cathode 46 is connected through the potential drop producing resistor 43 to a variable contact 95 which may be moved along the resistor 94 from the negative end toward: the positive end for varying the potential applied to the rectifier circuit in opposition to the signal controlled potential existing in the resistor 43 whereby the delay in response to signals may be varied. Any other suitable variable means may be used for this purpose.

Withthis arrangement, the contacts 92- and 1 93: may be varied independently or by unicontrol means such as a single control knobindicated at 96' and connected to move the contacts in opposite directions, as viewed in the drawings, through a connection indicated by the dotted lines 91 and.

When connected as shown, the reception is sharp when thecontact 93 is: moved to the extreme lower end of the resistor 9| as viewed in the drawings and is broad when: the contact is moved to the opposite extremity of this resistor, the contact 92 being moved simultaneously in the opposite direction. The fixed potential drop producing resistor in the rectifier circuit may be included in certain cases as indicated at 50. It will be seen that for any condition of operation, the contact 95 may also be adjusted to change the effect of the control potentials to increase or decrease the response and also to vary the degree of delay in the response at low signal levels. The circuit shown in Fig. 4 operated otherwise in the same manner as that shown and described in connection with the preceding figures and serves to provide for varying the relation between the automatic control Voltage applied to either of two control circuits in a system which provides in effect an automatic tone control which produces the low frequency response of the receiver when the selectivity of the receiver is increased and vice versa, so as to maintain the proper acoustic balance for all degress of selectivity between the high and the low frequency ends of the audio frequency or acoustic range of the output signals.

The automatic tone compensator tube used in conjunction with the automatic selectivity control circuit provides for changing the audio frequency characteristic of the receiving system to such an extent and in a direction to prevent an excess of low frequency response in the output of the system and would ordinarily cause a notable preponderance of bass in the signal output, particularly when receiving weak signals.

I claim as my invention:

1. In a radio receiving system, the combination of means for varying the selectivity of said system, and means for maintaining the acoustic balance of the audio frequency output characteristic thereof substantially constant with variations in selectivity, said last named means including audio frequency tone control means for attenuating signals in the low frequency end of the audio frequency range simultaneously with the eifective attenuation of signals in the high frequency end of the audio frequency range by operation of said selectivity control means.

2. In a radio receiving system, the combination with a high frequency signal circuit and a low frequency signal circuit, of means for attenuating signals in the lower audio frequency range in the low frequency circuit, selectivity control means operative to attenuate signals in the high audio frequency range in the high frequency signal circuit, means responsive to variations in signal amplitude for providing a variable control potential, and means for applying said potentials to said first and second named means to control the same in accordance with variations in signal strength.

3. In a radio receiving system, the combination with a high frequency signal circuit and a low frequency signal circuit, of means for attenuating signals in the lower audio frequency range in the low frequency circuit, selectivity control means operative to attenuate signals in the high audio frequency range in the high frequency signal circuit, means responsive to variations in signal amplitude for providing a variable control potential, and means for applying said potentials to said first and second named means to control the same in accordance with variations in signal strength, said last named means including a single control device operated to selectively apply said potentials to said first and second named means.

4. In a radio receiving system means responsive to variations in signal strength providing a variable control current, an impedance device,

automatic selectivity control means, automatic tone control means connected with one terminal of said impedance means, a gain control signal amplifier connected with a second terminal of said impedance means, and means for variably applying said signal control current to said impedance means to provide controlling potentials for said automatic selectivity tone and gain control means.

5. In a radio receiving system, a selectivity and tone control circuit, a gain control circuit, a potentiometer impedance connected between said circuits, means for variably applying signal controlled currents to said potentiometer impedance means, thereby to provide controlling potentials for said circuits, and means responsive to variations in signal strength for providing said currents, said means including a rectifier circuit, a voltage drop producing impedance in said circuit, means providing a fixed source of potential in said circuit, means for passing the signal controlled currents through said first named impedance in the rectifier circuit, and means providing tap connections on said potentiometer impedance for said rectifier circuit whereby said potentiometer impedance is variably included in said rectifier circuit.

6. In a radio receiving system, the combination with a high frequency circuit and a low frequency circuit, of means for varying the selectivity of said high frequency circuit whereby the high frequency end of the low frequency output characteristic of said system is reduced with increased selectivity, means for simultaneously reducing the low frequency end of said low frequency output characteristic correspondingly to provide a predetermined acoustic center of balance in said low frequency output characteristic.

'7. In a radio receiving system, the combination of a high frequency amplifier device having a control grid and a cathode, a rectifier device,

means for causing current to fiow through said nection between one of said terminal ends and said grid, a movable tap connection on said resistor connected with said cathode, a resistor in said connection having a lower resistance than that of said resistor element, and means for simultaneously varying the selectivity of said;

system and the low frequency end of the audio frequency response characteristic comprising a pair of control tubes each having a grid connected with the other terminal end of said resistor element.

8. In a radio receiving system, the combination of a high frequency amplifier device having a control grid and a cathode, a rectifier device, means for causing current to flow through said rectifier device in response to signals above a predetermined amplitude, a resistor element having a tap between its terminal ends connected with the rectifier anode, means providing a connection between one of said terminal ends and said grid, a moveable tap connection on said resistor connected with said cathode, a resistor in said connection having a lower resistance than that of said resistor element, means for simultaneously varying the selectivity of said system and the low frequency end of the audio frequency response characteristic comprising a pair of control tubes each having a grid connected with the other terminal end of said resistor element, and means for simultaneously moving said first named tap connection and said moveable tap along said resistor element in opposite directions.

9. In a radio receiving system, the combination of a high frequency amplifier device having a control grid and a cathode, a rectifier device, means for causing current to flow through said rectifier device in response to signals above a predetermined amplitude, a resistor element having a tap between its terminal ends connected with the rectifier anode, means providing a connection between one of said terminal ends and said grid, a moveable tap connection on said resistor connected with said cathode, a resistor in said connection having a lower resistance than that of said resistor element, means for simultaneously varying the selectivity of said system and the low frequency end of the audio frequency response characteristic comprising a pair of control tubes each having a grid connected with the other terminal end of said resistor element, means providing a signal variable source of potential in circuit with said rectifier, a power supply circuit and a loudspeaker field winding providing a filter choke coil in said last named circuit and a source of delay potential in opposition to said last named source of potential in circuit with the rectifier.

WINFIELD R. KOCH. 

